The present application relates to a battery pack and a method for detecting the degradation of a battery. In particular, the present application relates to a battery pack that can detect more correctly the degradation level of a degraded battery and a method for detecting the degradation level of a battery.
In recent years, battery packs using lithium ion secondary batteries have been widely used as power sources for electronic mobile apparatuses, such as note-type personal computers, mobile phones, and personal digital assistants (PDAs), and high power apparatuses, such as in-vehicle apparatuses and electric power tools. A lithium ion secondary battery has advantageous features, such as light weight, high capacity, ease of remaining capacity detection, and long cycle life.
A battery pack is a consumable. Secondary battery housed in the battery pack will be degraded by an increase in number of times used, and time-dependent degradation caused by leaving it standing for a long period. Thus, the more the secondary battery degrades, the more the available time of the secondary battery, which is from a fully charged state to a discharged state, decreases. For this reason, it is desirable to detect correctly the available time and capacity of a battery, which change with use or nonuse, and display a remaining capacity on an electronic apparatus. The electronic apparatus is typically designed to inform the user of the remaining capacity of a battery pack or an internal battery with a display screen, a lamp, or the like. If there is a large difference between the actual remaining capacity and the detected remaining capacity (i.e., remaining capacity of which the user is notified), problems may be caused. For example, the electronic apparatus in use is suddenly powered off. In addition, the amount of charge decreases as a result of being determined as a full charge in spite of without reaching to a fully charged state.
To determine the internal resistance of a secondary battery has been known in the art, there is a common method that calculates a direct current (DC) resistance from a voltage difference between a closed circuit voltage (CCV) and an open circuit voltage (OCV) and a current value in a closed circuit state.
For example, Japanese Unexamined Patent Application Publication No. 2008-41280 discloses a method for measuring the degradation degree of a secondary battery. That is, after the user repeats the charge of a secondary battery ten or more times by turning the charge on/off, an open circuit voltage is measured in a state where the charge of the battery is turned off. Then, the degradation degree of the secondary battery is calculated using the measured open circuit voltage together with both a previously measured closed circuit voltage and a charging current value.
Japanese Patent No. 3930777 also discloses a method for measuring the degradation degree of a secondary battery. That is, the net resistance value of a secondary battery is calculated from a discharging current periodically measured during a period of the flow of incoming current and a terminal voltage corresponding to the discharging current. In addition, the polarization resistance value of the battery is calculated from the discharging current and the terminal voltage and the degradation degree of the battery is then calculated using the net resistance value and the polarization resistance value.